Tim Willey

Adaptive Construction & Open Firing

Stacks Image 9

The Firing Stack
As a basis for experimentation, I have often used what I call a firing stack. It uses very little wood-fuel is easy to build and burns ferociously, testing to the limit any clay body you care to through in it!

A long time ago now, I worked with the Experimental Firing Group based at Leicester University’s Department of Archaeology . It was great fun, trying to recreate the firing technologies of the past and the research and findings of the group drove my interest in adapting historic technologies for contemporary practice.

One of the most important lessons learned during this time was the vital role that clay minerology and tempering (opening materials such as grog and sand) made to the success of often, very rapid firings.

At about this time, some fascinating research was being carried out by Patricia May and Margret Tuckson in Papua New Guinea, where they had an almost unbroken ceramic tradition which could be traced back for thousands of years. The research was comprehensively published in
‘The Traditional Pottery of Papua New Guinea’ and I remember seeing amongst its pages a very small image of an intriguing structure which I now call the firing-stack. A few lines of text also mentioned that the stack was used for a single pot and that the pots were not pre-heated!

Now this, to me, was amazing, as we know, a slow preheat of unfired pottery, is essential: that critical period from ambient temperature to about 200c must be tightly controlled, to allow moisture (present, even in the driest of pots), to escape without blowing its way out! The stack was also a paradigm of an adaptive technology where, no doubt, over the millennia, the structure was developed alongside highly adapted clay-bodies.

Over the years and with very little to go on, I’ve interpreted and adapted this stack structure to the point where I’m now only making very subtle changes to it’s architecture and choice of fuel.

The potters of Papua New Guinea knew a thing or two about clays and tempering clays, as indeed did all potters from ‘open-firing’ traditions, but it took me a long-long time to even come close to a clay that could stand the super-rapid temperature rise of the firing-stack without the need to pre-heat or pre-fire.

It’s not surprising that so many contemporary techniques that fire relatively quickly: pit firing, drum kilns, raku firing, paper kilns etc. all tend to use pre-fired (biscuited) pots. Some ‘off the shelf’ clays might work with rapid open-firing, but I must admit, I’ve had very little success.

So, whilst I made plenty of firing-stacks (my first was in the 80s) I had very few pots that remained intact and I pretty much kept the firing-stack experiments to myself whilst working - off and on - towards a clay that matched its performance.

There was never a eureka moment (although I thought there might be) instead, it was just a case of repeated testing until, over the years, a systematic approach started to develop.

I’ve included some details on how the clay formulation evolved, but first here are the dimensions of the stack and a few important notes:



Stacks Image 37
Firing Stack Construction

Height of stack 600mm
Length of wood fuel 500mm
Cross-section of wood fuel approx: 45x45mm
Base: fire brick or dry earth

Notes:
  • The pots are dried thoroughly
  • The pots are not pre-fired
  • the wood fuel is bone dry
  • Inner fuel is angled to create 'nest' for supporting pot
  • Dry leaves or paper is used to start fire
  • A slight breeze is okay but not a strong wind
  • The stack is ignited from the top
  • Pots are wrapped in paper for protection
  • Pots are placed on top when fire is underway
  • Falling wood fuel is eased back into the stack with a (long) fireproof garden hoe
  • I wear full PPE appropriate to hot working
Note: Open-firing is dangerous - I always wear PPE appropriate for hot working.